Priscilla Barielle[]
Although he was bribed to be with her, over time Aldebaran realized that he really wanted to protect her, and although he is often beaten by Priscilla, he continues to accompany her. He dislikes it when Priscilla uses his full name, getting unreasonably angry and snapping back at her. Despite that, he's perfectly willing to give his life for her safety and well-being as shown when he died 1000 times trying to convince her not to go to Vollachia and risk her safety. In the end, he gave in and is currently accompanying her to the Empire.
Heinkel Astrea[]
The two seem to hold mutual respect for each other. However, in Arc 8, he would show that he had no qualms with using Heinkel as a sacrificial pawn if it was deemed necessary.
Schult[]
Schult and Al both care about each other, since Aldebaran suffers a lot when Priscilla hurts him.
Natsuki Subaru[]
As fellow Japanese natives, the two often find it relatively easy to talk to each other, both understanding the same references and language. That said, due to his affiliation with Priscilla, Subaru would have difficulty trusting him at first. However, he would come to trust Al with time.
Al seems to have a strange understanding of Subaru. In Arc 5, Al would be the one to back up Subaru when regarding who should speak over the speakers to Priestella, informing the others that Subaru was not some Superman who could carry the burdens of everyone. Ironically, however, this would be what gave Subaru the courage to speak for the crowd. Al would warn him what this means, and by the time Subaru was done speaking, Al would be left in a strange mood.
Later, in Arc 7, when Subaru was struggling with how to handle Rem’s words, Al would stop him from busting his head open, and he would be the one to encourage Subaru to become a hero in her eyes again, even deciding to back up Subaru while he did so. He would also be extremely concerned for Subaru’s wellbeing after learning of Spica’s identity, not to mention the events of Chaosflame. Speaking of which, he seems to immediately realize that Subaru was in the center of the giant shadow, and later, he comments on Subaru being alive because the world hadn’t ended.
In Arc 8, he would state his belief in Subaru being the hero the world needed, going out of his way to pave a road for his arrival. Where Subaru would be the one to save everyone, Al would be the one to dirty his hands.
Emilia[]
Al would first talk to Emilia in Arc 5 over a pair of Communication Mirrors. Al would state his concern over Emilia’s actions, but Emilia would trust Al to get her message to Subaru. After their short conversation, Emilia would find herself being reminded of Subaru when talking to Al, though she wouldn’t understand why.
Before this, however, during a joint meeting between the Priscilla and Al, Anastasia and Julius, and the reporter Shorty Megan, Al would stand up for Emilia when the others would talk negatively about her.
Rem and Ram[]
Al seems to be acquainted with Ram as he mistook Rem for her older sister during the third failed loop of Arc 3. Throughout their following short interaction, Al expressed his shock at realizing he was speaking with Ram's sister, indicating he only knew of her existence and not of Rem's. Soon after, he turned extremely hostile though managing to contain himself in mere seconds. After Rem and Subaru departed from Priscilla's manor, Al seemed to have gained somewhat of a grasp on the situation; a situation that made him want to vomit.
Rem seems to sense something negative towards her from Al's gaze.
Garfiel Tinsel[]
Al would first meet Garfiel when the boy was searching for the missing Subaru in Arc 5, with him mistaking Al’s scent for Subaru. Garfiel would be openly distrusting of Al in City Hall, and during the talks of who should speak to the people of Priestella, Garfiel would end up angered when Al said that Subaru couldn’t do it. However, Al would point out that Garfiel’s habit of calling him Boss wouldn’t make him some superhero who could solve any issue, causing Garfiel distress as he was telling him of the pressure Subaru was being put under.
Cecilus Segmunt[]
Al and the infantilized Blue Lightning would temporarily team up during the Great Disaster. Al would often find himself annoyed with Cecilus’s antics, especially so when Al was left hanging off of the side of a building and left to just die. However, Al also recognized his strength as something necessary. Later, the two would fight together against Arakiya, who would be suffering under the effects of having devoured more of Muspel. Thanks to Al, Cecilus would manage to defeat Arakiya, sacrificing his own sword to do so.
Cecilus seems to realize Al’s power to a certain extent based on his intuition and Al’s behavior. Furthermore, he would also compare Al to Subaru, though he would point out their differing methods.
Arakiya[]
Al and Arakiya crossed paths roughly 7 years before the start of the main story, during the gladiator upbringing that took place on the gladiator island Al was situated at. Upon first meeting Al, Arakiya expressed her interest in his face, stating it "wasn't bad". The two were forced to form a short alliance in order to fight another gladiator by the name of Hornet, who was guarding the drawbridge which connected the island with the mainland and was preventing the Imperial Army from entering the island and subduing the uprising. After their fight, Al expressed his immense happiness for finally being able to save a beautiful silver haired girl, as he once already had the chance to do so, but failed.
However, the two would be forced to point their weapons at each other when they met again in Guaral in Arc 7. Though he and Priscilla managed to bring her down, Al would be disappointed to have to come to blows with her after all this time. Even more unfortunately, Al would be forced to fight her again in the following arc, assisting Cecilus to bring down the suffering girl.
Groovy Gumlet[]
The two would meet during the Great Disaster, where they would be forced to work together. Al and Groovy seem to get along rather well in spite of Groovy’s constant cursing, with the latter even apologizing for jumping to the top of a building and leaving Al on the ground. Ultimately, Al would entrust the handling of Eugard Vollachia to the hyena-man.
Capella Emerada Lugunica[]
The two would meet underneath City Hall in Priestella. Impressed with the trap, Capella would engage in combat with the man, and even find his actions odd, hearing him speak of things as if he had prior knowledge. Because of this, combined with his brutality against her, she would find Al interesting. During the fight, Capella would tease him by transforming herself closer and closer to Priscilla’s image, using the slightest hints of Al’s behavior to do so. Unfortunately, Al could not land a decisive blow against her, and she was able to escape.
Spica[]
At first, he would find Spica’s attachment to Subaru rather entertaining if not odd, even becoming curious of why Subaru was so wary of her. Though she seemed not to like him. Later, however, when he learned of her identity as a Sin Archbishop of Gluttony, his rage would peak at her, and he was even willing to cut her head off right then and there. Following the events in Chaosflame, he would decide not to harm her for Subaru’s sake, though he would remain extremely wary of her.
Satella[]
It is heavily implied that Al knows Satella personally. In Sunlight on the Water, Al encounters an illusion of a shadowy figure wearing a shadowy dress, which Al was unable to bring himself to harm even though he tried some 100,000,000 million times. Unable to do anything, he was gripped with fear, and tried to retreat as it attempted to touch him lovingly with its fingers. Fortunately, Priscilla destroyed it with her Yang Sword, yet this caused him to be filled with rage, though she easily dealt with him by smacking his helmet with her sword. Although he was still scared somewhat, he was relieved that the shadowy figure was fake.
In The Sword Wolf of the Scintillating Flame, Al states that at some point in his life, a beautiful silver haired girl appeared before him when he was about to die. He also states that silver hair brings him bad memories about how a "trash, stupid useless person he is" due to how he failed to save the one he cherishes the most.